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PYROMETRY                                              441
parent quartz-glass tubing is manufactured, but on account of its cost is not used industrially for thermocouple protection. Fused quartz affords good protection up to 1,050C. in an oxidizing atmosphere free from alkalies. The material is somewhat pervious to hydrogen and probably to other reducing gases, but is not -pervious at this temperature to oxygen or carbon dioxide. Any reducing gas within the protecting tube^of a rare-metal couple is disastrous, particularly when the tube contains silica.^ The silica is reduced to silicon which is readily absorbed by platinum. Above 1,OSOC., and even at lower temperatures after prolonged heating, quartz devitrifies and crumbles away. Quartz tubes withstand sudden changes of temperature without breaking. Heavy sintered quartz tubes, with walls 1 or 2 cm. thick, are sometimes used for extra protection, for example against acid fumes.
Porcelain is used primarily for protection of rare-metal couples. Previous to 1914 a highly refractory porcelain known as Marquardt was imported from Germany. A better grade of this material was developed through the research work of the Bureau of Standards and is now manufactured by Stupakoff under the trade name "Usalite," and by Engelhard under the trade name "Impervite." These two porcelains have a melting point above that of platinum. However, they are impervious to gases only when glazed. The softening point of the glaze used is about 1,300C. If the tubes are glazed on the outside only they are serviceable as pyrometer protection tubes up to 1,500C. The glaze on the German tubes softens at 1,200C. The insulating tubes are not glazed. Pyrometer porcelain for use at lower temperatures is made by Guernsey Earthenware Co., Herold China <fe Pottery Co., and other American companies, the advertisements of which appear in the trade journals, also by the Royal Worcester Co., England, and a porcelain known as S. C. P. is made by a Japanese firm. The glaze on the Japanese porcelain softens at about 1,100C. and its maximum serviceable temperature is about 1,250C., which is higher than that of the, Berlin porcelain of similar grade, or the Royal Worcester. We do not have data on similar grades of American porcelain which have been developed during 1918-1919. An unprotected porcelain tube suddenly thrust into a furnace at 1,000C. will usually break. If it is inserted slowly, however, there is little danger of breakage.
In permanent installations quartz and porcelain tubes and also the iron or chromel tubes of base-metal couples are frequently further protected by heavy outer tubes of fire clay, carborundum, graphite, etc. The outer tube is usually cemented in place in the furnace wall forming a well into which the couple is inserted. In case the outer lube introduces too large a temperature lag or where there is danger of lowering the temperature of the hot. junction of the couple by conduction of heat through the heavy tube, the latter is made open at both ends. The couple and its smaller protecting tubes is so mounted that the hot junction projects beyond the end of the outer protecting tube a few centimeters.
Carborundum, or silicon carbide, SiC, is used for outer protecting tubes. It has u high-thermal conductivity (about twice that of silica), a low coefficient of expansion (about one-half that, of fused alumina), and a high mechanical strength. When heated in an oxidizing atmosphere, oxidation begins at about 1,200C. Gases except chlorine- do not act on carborundum. Carborundum reacts with practically all metals at high temperature. Platinum must be thoroughly protected from the carborundum by a gaslight inner tube. Carborundum or silfrax, which is pure carborundum of very fine crystalline structure, is highly satisfactory for ordinary furnaee work. It is sometimes used in molten glass and open-hearth slag, although bawio sings attack it readily.
Cast nichromc or chromel tubes are used extensively for protection of both base-metal arid rare-metal couples. These tubes resist oxidation remarkably well and although much more costly than iron tubes, in many processes, their longer life war-